1. Field of the Invention
This invention relates to a fire detector and a fire alarm system employing the same, and more particularly to a fire detector and a fire alarm system wherein a plurality of fire detectors are connected across a pair of lines leading to a signal station and which is capable of detecting, at the signal station, removal of a detector head or heads of the associated detector or detectors and yet capable of keeping the system operative so that the signal station may receive a possible fire alarm signal even after removal of the detector head or heads.
2. Description of Prior Art
In general, a fire alarm system (hereinafter referred to as "alarm system") has a plurality of fire detectors (hereinafter referred to as "detector") sequentially connected in parallel to a line and a signal station adapted to receive a fire alarm signal from the respective detectors to give an alarm.
Since the alarm system is not actuated in a normal condition and works only at an exceptional, abnormal time when a fire breaks out, it should be fully prepared against emergencies and have high reliability so that it can give an accurate alarm when a fire starts. To assure high reliability of the system, not only the detectors but also the line per se should have a sufficient reliability. To this end, it is required to immediately detect possible breaking of the line and alarm such breaking so as to enable quick repair of the breaking.
Therefore, it has been proposed to provide a fire alarm system equipped with a breaking detecting means at a signal station and capable of monitoring line conditions. In this system, a small current for monitoring flows constantly or periodically through the line via a terminating element. The breaking detecting means detects possible breaking of the line from a phenomenon of shut-off of the current due to the breaking. The breaking detecting means is formed, for example, of a semiconductor.
Each of the detectors employed in the fire alarm system is comprised, as illustrated in FIG. 1, of a socket 2 fixed to a ceiling etc. of a building and connected to the line leading to the signal station and a detector head 3 including a detecting means for detecting smoke, heat, etc. The detector head 3 is attached to the socket 2 by engaging a contact blade 5 of the head 3 with a holder member 4 having a holding resilient member 4b provided on the socket 2. Thus, the detector head 3 is formed detachable from the socket 2. This is very convenient for installation of the detector, maintenance check, exchange, etc. of the detectors and improves efficiency of these operations. However, due to this removable formation of the detector head, the detector head is unfortunately sometimes removed by intruders or thoughtless persons. A problem is that the signal station cannot detect the removal of the detector head because a plurality of detectors are connected in parallel with each other in the fire alarm system.
To solve this problem and assure high reliability of the system, it has been proposed to detect removal of the detector head, utilizing the aforesaid breaking detecting means. More specifically, in this improved system, the line is put into a breaking condition when any one of the detector heads is removed from the associated sockets and the signal station is adapted to detect the breaking of the line.
For instance, as illustrated in FIG. 2, a line 7 leading to a signal station 6 is connected, through a contact means 9 which is provided in each of detectors 1 and adapted to conduct when the detector head 3 is fitted to the socket 2, to a succeeding detector 1 and a terminating element 8 comprised of a resistor 8a etc. is connected at the end of the line 7 so that the signal station 6 may detect removal of the detector head. As illustrated in FIGS. 1, 3A and 3B, the contact means 9 of the detector 1 is comprised of a holder member 4, a resilient member 11 provided adjacently to the holder member 4 and the contact blade member 5, and the contact blade member 5 is interposed between and in contact with the holder member 4 and the resilient member 11 when the detector head 3 is attached to the socket 2 to conduct the line 7 for supplying a power source to a detecting portion 10. On the other hand, when the head 3 is removed, the holder member 4 is isolated from the resilient member 11, rendering the contact means 9 non-conducting to disconnect the line 7. Thus, removal of the detector head 3 is detected.
However, this detector and the fire alarm system employing the same have such a disadvantage in practical use that when any one of the detector heads 3 is removed, the line 7 is put into a breaking condition and all the detectors succeeding the detector whose detector head has been removed become inoperative. This system is too much adapted for detection of removal of the detector head to perform a fire detecting function which is essential to a fire alarm system. Thus, this system has a fatal defect as a fire alarm system.
Further, there has been proposed a detector having a contact means on a socket which is kept in a non-conducting state when a detector head is attached to the socket and adapted to conduct a short-circuit a line when the head is removed. This detector, however, has a disadvantage that a signal for indicating removal of the detector head cannot be distinguished from a fire alarm signal because both the signals are caused by short-circuiting of the line. In addition, this detector lacks reliability of the system and is not practicable because removal of one detector head hinders alarming operating of other detectors.
Where a fire alarm system has a special testing signal line for testing operations of detectors, either of the preceding two proposals may be carried out for detecting removal of a detector head. However, this system cannot be applied to a fire alarm system having no test signal line. Therefore, a special line must be provided at a time of installation of a fire alarm system, which increases an installation cost. Thus, this system is not always desirable and it is unpractical, in especial, when an area to be covered by the system is considerably large.
As described above, none of the foregoing fire alarm systems can effect detection of removal of the detector head, utilizing disconnection or short-circuiting of the line without causing hindrance to alarm operations of other detectors. Thus, a detector and a fire alarm system which is capable of solving the problems involved in the conventional detectors and systems and capable of detecting removal of the detector head with high reliability has not been proposed.
The present invention has been made in view of these facts and achieved based on a finding that such a short time, as several micro-seconds to several seconds, will suffice to detect disconnection of a line. Such a momentary time required for detection of the disconnection is negligible for reliability of the system because there is substantially no chance that a fire will break out during such a momentary time. More specifically, the present invention is so formed as to detect removal of the detector head by momentarily disconnecting the line during a time required for the detecting in the course of or after removal of the detector head. Thus, the invention provides a novel fire detector and fire alarm system which can markedly enhance reliability of the system without causing hindrance to the succeeding detectors.